A vehicle powered by an engine that combusts a mixture of air and petrol-based fuel is customarily outfitted with an exhaust aftertreatment system to diminish the amount of unwanted gaseous emissions contained in the engine's exhaust flow. The primary emissions targeted for removal include carbon monoxide, unburned and partially burned hydrocarbons (HC's), and nitrogen oxide compounds (NOX) comprised mostly of NO and NO2 along with nominal amounts of N2O. The concentrations of these emissions in the exhaust flow constantly fluctuate in response to compositional changes in the mixture of air and fuel fed to the engine. The rigors of stop-and-go traffic and frequent accelerations and decelerations, for example, cause the air to fuel mass ratio of the air/fuel mixture to continuously oscillate above and below a predetermined target value.
The exhaust aftertreatment system defines a contained passageway from the engine to a tailpipe opening for communication the exhaust flow away from the engine and ultimately to the atmosphere. The passageway guides the exhaust flow through one or more specially catalyzed flow-through components that are able to convert changing concentrations of the unwanted gaseous emissions into more innocuous substances. The particular architecture of the exhaust aftertreatment system and the catalysts used depends largely on the normal expected operating window of the air to fuel mass ratio of the air/fuel mixture (i.e., whether the mixture of air and fuel is stoichiometric, lean, or rich). These systems seek to oxidize carbon monoxide and HC's (to carbon dioxide and water) and to reduce NOX (to nitrogen and water).
The combustion of a mixture of air and fuel that, over time, averages a stoichiometric mass ratio of air to fuel typically produces a low-oxygen content exhaust flow that contains a desired balance of the unwanted gaseous emissions. A relatively low but not insignificant amount of carbon monoxide and HC's are present and, along with other fuel-derived compounds (i.e., hydrogen), provide reductive activity for NOX conversion. The low amount oxygen present, on the other hand, is sufficient to provide oxidative activity (along with NOX) for carbon monoxide and HC oxidation but not great enough to diminish the reductive activity of those compounds. The long-standing practice to treat such an exhaust flow has been to equip the engine with an exhaust aftertreatment system that includes a catalytic converter. The catalytic converter includes a support substrate loaded with a TWC to promote intimate contact between the exhaust flow and the TWC. The reaction balance of reductants (CO, HC's, H2) and oxidants (O2, NOX) in the exhaust flow permits the TWC to concurrently reduce NOX and oxidize carbon monoxide and HC's through various coupled catalytic reactions. A well-known example of a TWC-loaded support substrate is a monolithic honeycomb structure made from stainless steel or cordierite and washcoated with alumina and a platinum group metal fine-particle mixture (the TWC) of platinum, palladium, and rhodium.
A specific and commonly employed exhaust aftertreatment system for a stoichiometric-burn engine is a split converter configuration that employs two spaced apart catalytic converters. A first catalytic converter is mounted to the engine's exhaust manifold near the engine compartment (the close-coupled position) and a second catalytic converter is positioned downstream from the first catalytic converter and underneath the vehicle (the under-floor position). The close-coupled catalytic converter immediately receives the exhaust flow from the engine which, during cold-starts, helps quickly heat the TWC to its light-off temperature. The under-floor catalytic converter supplements the catalytic activity of the close-coupled catalytic converter during warmed-up conditions and is particularly suited to reduce NOX breakthrough when air to fuel mass ratio fluctuations are experienced in the air/fuel mixture. The split converter configuration, besides offering enhanced emission control, also generally requires less overall rhodium content to achieve effective NOX conversion when compared to system designs which utilize only a single close-coupled catalytic converter.
Some of the reductants present in the low-oxygen content exhaust flow can, however, react with NOX over the TWC in the close-coupled catalytic converter to passively generate ammonia. The detailed reaction chemistry at the catalyst surface is rather complex. But, in general, the lack of oxygen enables NO to participate in secondary alternative reactions with carbon monoxide and/or hydrogen to form ammonia according to the overall reaction equations:2NO+2CO+3H2=2NH3+2CO2  (1)2NO+5H2=2NH3+2H2O  (2)The extent of ammonia formation is affected by a number of engine operating parameters including, for example, the air to fuel mass ratio of the air/fuel mixture combusted in the engine, the temperature of the exhaust flow, and the exhaust flow gas hourly space velocity. A stoichiometric or rich air/fuel mixture typically results in greater amounts of ammonia being formed over the TWC than a lean air/fuel mixture. Ammonia formation over the TWC also generally peaks during engine operating conditions consistent with vehicle acceleration events.
The generation of ammonia from native NOX and exhaust reductants over the close-coupled catalytic converter may increase the likelihood of ammonia slip to the atmosphere. The placement of a PGM-based oxidation catalyst downstream from the under-floor catalytic converter to remove residual ammonia from the exhaust flow adds expense and complexity to the exhaust aftertreatment system. A continuing need therefore exists to develop methods and exhaust aftertreatment system designs that can treat the exhaust flow emitted from an engine that is combusting, on average, a stoichiometric mixture of air and fuel and, at the same time, reduce the possibility of ammonia slip to the atmosphere.